Devices or systems for providing a humidified gases flow to a patient for therapeutic purposes are well known in the art. Systems for providing therapy of this type (for example respiratory humidification) have a structure where gases are delivered to a humidifier chamber from a gases source. As the gases pass over the hot water, or through the heated, humidified air in the humidifier chamber, they become saturated with water vapour. The heated humidified gases are then delivered to a user or patient downstream from the humidifier chamber, via a gases conduit and a user interface. The gases delivery system can be a modular system that has been assembled from separate units, with the gases source being an assisted breathing unit or blower unit. That is, the humidifier chamber/heater and the blower unit are separate (modular) items. The modules are in use connected in series via connection conduits to allow gases to pass from the blower unit to the humidifier unit. Alternatively, the breathing assistance apparatus can be an integrated system, where the blower unit and the humidifier unit are contained within the same housing in use. In both modular and integrated systems, the gases provided by the blower unit are generally sourced from the surrounding atmosphere. A third general form of breathing assistance system, which is typically used in hospitals, is one where the breathing assistance system receives at least a portion of the gases which it uses from a central gases source, typically external to the area of use (e.g. a hospital room). A gases conduit or similar is connected between an inlet which is mounted e.g. in the wall of a patients room (or similar). The gases conduit is either connected directly to the humidifier chamber in use, or a step-down control unit or similar can be connected in series between the gases inlet and the humidifier chamber if required. This type of breathing assistance system is generally used where a patient or user may require oxygen therapy, with the oxygen supplied from the central gases source. It is common for the pure oxygen from the gases source to be blended with atmospheric air before delivery to the patient or user, for example by using a venturi located in the step-down control unit. In systems of the type where at least some of the gases are delivered from a central source, there is no need for a separate flow generator or blower—the gases are delivered from the inlet under pressure, with the step down control unit altering the pressure and flow to the required level.
An example of a known, prior art, type of modular system using atmospheric gases only is shown in FIG. 1.
In typical integrated and modular systems, the atmospheric gases are sucked in or otherwise enter a main ‘blower’ or assisted breathing unit, which provides a gases flow at it's outlet. The blower unit and the humidifier unit are mated with or otherwise rigidly connected to the blower unit. For example, the humidifier unit is mated to the blower unit by a slide-on or push connection, which ensures that the humidifier unit is rigidly connected to and held firmly in place on the main blower unit. An example of a system of this type is the Fisher and Paykel Healthcare ‘slide-on’ water chamber system shown and described in U.S. Pat. No. 7,111,624. A variation of this design is a slide-on or clip-on design where the chamber is enclosed inside a portion of the integrated unit in use. An example of this type of design is described in WO 2004/112873.
One of the problems that has been encountered with systems that provide a flow of heated, humidified gases to a patient via a gases conduit and an interface is that of adequately controlling the characteristics of the gas. Clearly, it is desirable to deliver the gas to the patient (i.e. as it exits the user interface) with the gas at precisely the right temperature, humidity, flow, and oxygen fraction (if the patient is undergoing oxygen therapy) to provide the required therapy. A therapy regime can become ineffective if the gases are not delivered to the patient with the correct or required characteristics. Often, the most desirable situation is to deliver gases that are fully saturated with water vapour (i.e. at substantially 100% relative humidity) to a user, at a constant flow rate. Other types or variations of therapy regime may call for less than 100% relative humidity. Breathing circuits are not steady-state systems, and it is difficult to ensure the gases are delivered to a user with substantially the correct characteristics. It can be difficult to achieve this result over a range of ambient temperatures, ambient humidity levels, and a range of gas flows at the point of delivery. The temperature, flow rate and humidity of a gases stream are all interdependent characteristics. When one characteristic changes, the others will also change. A number of external variables can affect the gases within a breathing circuit and make it difficult to deliver the gases to the user at substantially the right temperature, flow rate and humidity. As one example, the delivery conduit between the patient or user and the humidifier outlet is exposed to ambient atmospheric conditions, and cooling of the heated, humidified gases within the conduit can occur as the gas travels along the conduit between the exit port of the humidifier chamber and the user interface. This cooling can lead to ‘rain-out’ within the conduit (that is, condensate forming on the inner surface of the conduit). Rain-out is extremely undesirable for reasons that are explained in detail in WO 01/13981.
In order to assist in achieving delivery of the gases stream with the gases having the desired characteristics, prior art systems have used sensors (e.g. temperature and humidity sensors) located at various positions throughout the breathing circuit. Thermistors are generally used as temperature sensors, as these are reliable and inexpensive. Humidity sensors such as the one described in U.S. Pat. No. 6,895,803 are suitable for use with systems that deliver heated humidified gases to a user for therapeutic purposes.
In order to achieve delivery of the gases to the patient at the correct temperature and humidity, it is necessary either to measure or sense the gases characteristics at the point of delivery, or to calculate or estimate the gases characteristics at the point of delivery from measurements taken from elsewhere in the system. In order to directly measure the gases parameters at the point of delivery, sensors must be located at or close to the point of delivery—either at the end of the patient conduit or within the interface. Sensors located at or close to the point of gases delivery will give the most accurate indication of the gases state. However, one consideration when designing a breathing circuit is to ensure that the components used in the breathing circuit can be repeatedly connected and disconnected to and from each other, with high reliability. Another consideration is to keep the weight carried by the patient in use to a minimum, and therefore it is desirable to keep the number of sensors at the patient end of the conduit to a minimum, or remove the need for these altogether. It is also desirable to keep the total number of sensors in the system to a minimum, in order to reduce costs and complexity (e.g. an increased number of electrical and pneumatic connections).
In order to overcome or sidestep the problem or trade-off of accurate measurement of the gases characteristics vs complexity vs cost vs weight carried by the patient vs reliability, sensors can be located at various other points within the system to measure the parameters of the gas at those points, and the readings from these sensors can be used by a controller to estimate or calculate the characteristics of the gases at the point of delivery. The controller then adjusts the output parameters of the system (e.g. fan speed, power to the humidifier chamber heater plate, etc) accordingly. One example of a system and method where this type of calculation is carried out is disclosed in WO 2001/13981, which describes an apparatus where there are no sensors at the patient end of the conduit. A, temperature sensor is located proximal to the heater plate in order to measure the heater plate temperature. The flow of gases through the humidifier chamber is estimated, and the appropriate power level for the heater plate is then determined by a central controller. The controller estimates the power supply to the heater humidifier plate, and the power required by the conduit heater wire for achieving optimal temperature and humidity of the gases delivered to a patient.
One possible disadvantage of systems and methods which estimate the gases characteristics (such as the system and method disclosed in WO 2001/13981) is that the estimations and algorithms used are not as accurate as is necessary. There are many variable factors that can detrimentally effect the accuracy of the calculation algorithms used by the controller. These factors may not have been taken into consideration when the algorithm was designed. For example, the apparatus and in particular the humidifier chamber can be subject to convective heat loss (‘draft’) which is created by external airflows, particularly in ventilated spaces. The flow velocities of the air vary in magnitude, direction and fluctuation frequency. Mean air velocities from below 0.05 m/s up to 0.6 m/s, turbulence intensities from less than 10% up to 70%, and frequency of velocity fluctuations as high as 2 Hz that contribute up to 90% of the measured standard deviations of fluctuating velocity have been identified in the occupied zone of rooms—for one example, see Volume 13, number 6 of HVAC&R Research—paper tided: ‘accuracy limitations for low velocity measurements and draft assessment in rooms’, by A Melikov, Z Popiolek, and M. C. G. Silva.
The system disclosed in WO 2001/13981 is unlikely to be able to provide the control precision necessary to control humidity accurately without substantial rainout occurring. A user or manufacturer may be forced to trade-off delivery of gases at a lower humidity level, against an increased possibility of rain-out, against the number of sensors used and their location in the breathing circuit. For example, when the incoming gas delivered to the humidifier chamber from the compressor or blower (particularly in an integrated blower/humidifier breathing assistance system) has an increased temperature, the chamber temperature should be accurately compensated to achieve the desired dew point. If the air coming into the chamber is warm and the air temperature is increasing with an increase in flow, then the inaccuracy of a set calculation algorithm will increase.
It should further be noted that prior art systems frequently measure/calculate and display the humidifier chamber outlet temperature. Displaying the temperature reading is often inadequate for a user to make an informed decision, as the temperature does not always directly relate to the gases humidity state. This is due to a number of factors, of which the following are examples, but not an exhaustive list.                1. High temperature of the incoming gas.        2. Very low or very high flow rate.        3. Cooling of the humidifier chamber by convection of the ambient air around the humidification chamber.        4. Mixing of outgoing and incoming gases inside the chamber.        5. Condensation of water at the chamber wall or connection tubes particularly at low ambient temperature conditions.        6. Problems with accurate temperature measurements at high humidity (the ‘wet bulb’ effect).        7. Variations in the level of the humidity of the incoming gas.        
Furthermore, a user may not always require gases warmed to body temperature and 100% humidity. A specific therapy regime may call for a high or 100% humidity level, but this can be undesirable for users who use a mask, as the conditioned gas with high humidity can feel uncomfortable for a user on their skin.
A further problem in system of this type can be outlined as follows: It is normal in systems such as those outlined above for the fan speed (modular and integrated units) or pressure/flow level (hospital, remote source units) to be set to a constant level, with the assumption that this will provide a constant flow rate throughout the system (or alternatively, if using a central gases source in the system, the flow rate of the incoming gases from the remote source is assumed to remain constant). A constant flow rate is desirable for the same or similar reasons as outlined above. A constant flow rate is also very desirable when using additional or supplementary oxygen, blending this with atmospheric gases. A constant flow rate will help to keep the oxygen fraction at the desired level.
As the gases characteristics are interdependent, a change in the flow rate may lead to a significant change in the humidity, temperature or oxygen fraction of the gases delivered to a user. However, the flow through the system may be affected by a number of different interdependent variables which are independent of the gases source (e.g. the speed of the fan). These can include increased (or decreased) resistance to flow caused by changes in the position of the user interface on a user, changes in the way the delivery conduit is bent in use, etc. The flow rate will also change if, for example, the interface is changed to a different size or shape of interface, or a different type of interface altogether.
There is therefore a need for a system and method which provides increased control precision for controlling the humidity, or temperature, or both, of the gases flow, while at the same time delivering gases to a patient at the correct temperature, humidity and pressure for effective therapy. There is also the need for a system which compensates for changes in the resistance to flow through the system during use in order to provide a substantially constant flow rate at the desired level.